I built a tub to hold water in the bottom of the planter using old coroplast election signs and half a tube of sealant. I wasn't sure about having the paint from the signs in contact with the water so I lined that with a few layers of plastic (garbage bags). And I installed an overflow port at one end using a bulkhead I had lying around from a previous project.
Above the top of this tub I placed a couple of unused lids from some storage boxes. I drilled a few small holes for drainage and made larger holes for small plastic containers that can extend down below the deck and into the water. I drilled several holes into the sides of those little containers. Now when the planter is filled with potting soil and the reservoir is filled with water, moisture will be wicked up into the potting soil and the plants will send their roots down to find it.